Make sure whatever drives you use are CMR/PMR not SMR, dig into the drive manuals & specifications before buying.
Drobos don’t like SMR drives.
The usual cautions, insert & remove disk packs only with the unit powered off.
Replace smaller disks with bigger ones hot, with the power turned on, one at a time & wait the eternity for rebuild before doing the next.
Take note that migration tends to be one-way, if you plan to move the disks back after your proposed method of moving the data, they won’t work right away, I’ve no idea if a reset will fix that, you might have to resort to diskpart > CLEAN ALL or a secure erase program, which is slow as molasses poured in Norway in January, as are rebuild times & transferring the data over USB or the network might be as fast, if you’re using new bigger drives anyway in the new units, particularly from the ones with USB 3.
The power supply will likely work, they’re all 12v & newer units tend to have higher rated PSUs than older ones, so the 5C brick should power older units, but the older bricks probably won’t power the 5C (likely to result in boot loops). I don’t think the barrel jack has been changed & I’m reasonably certain the polarity hasn’t either.